Factory Five Racing IRS Conversion - Independent Thinking

Live axles are great for drag racing. The suspension is easy to tune, the parts are strong, plentiful, and inexpensive; for straight-line racing, the live axle is hard to beat. Put the same rearend in an uphill righthand hairpin, however, and your suspension is going to cringe, which can leave you in the gravel. For road racing, there is nothing better than an IRS or independent rear suspension. Giving the drive wheels the ability to articulate to a changing road surface independent of either side gives you an advantage over the traditional live axle. The problem comes in converting a straight axle car to IRS.

Part of what changed the Mustang from a "secretary" car into the most successful performance platform in the world was its success on both straight and twisty tracks. Vintage road racing is full of early Mustangs, making the aftermarket full of great race parts. The proven performance of an IRS swap can be found for the first pony cars, but they definitely are on the pricey side, starting at around $5,000 for a basic kit. For the budget enthusiast who doesn't want to drive a Miata, that cost tends to push the IRS swap out of reach. When we decided to add an IRS to this '62 Mercury Comet Wagon project, we knew this would be a challenge, especially trying to make it happen on a budget of less than $3,000.

The early Falcon/Comet platform is what the Mustang was developed on, but there are a few key dimensions that changed, specifically the width. The Mustang is 1.5 inches wider than the '60-'63 Falcon, so even the aftermarket IRS won't be an exact fit. After about eight months of researching, we found an answer—Factory Five Racing. FFR builds some of the coolest reproduction kits on the market, the MK-series being the Cobra roadster platform, currently in the fourth generation, the MKIV. When you order an MKIV, you have the option of live axle or IRS. The FFR IRS uses a few factory components from the '89-'97 V-8 Thunderbird installed into FFR's tubular IRS cradle that is designed to match up to the MKIV chassis. The Thunderbird IRS is often used for IRS swaps for trucks, but the issue with using it as-is in the Comet is the width. The track width on the Comet is a scant 54.5 inches, where the T-bird is rocking 60.2, which would put the hubs past the edge of the rear quarter-panel. The Factory Five IRS cradle also uses the 60.2-inch width, unless you opt for the pin-drive setup, which is designed for using knock-off adapters. This narrows the overall track width to 54.25 inches, and that we can deal with.

Once we had the basic plan, we called the guys at FFR and discussed it with them. While they thought we were off our rockers, they provided us with the necessary dimensions that we would need to ensure the system maintained the proper geometry. The upper mount is separate from the lower cradle; it has to be installed a specific distance from the lower cradle, centered above it, and at a specific distance to the front of the cradle. This keeps everything from binding up at full articulation. We ordered the pin-drive kit, which comes with new axle-shafts, tubular upper and lower control arms, parking brake cables, and all of the bolts. We also ordered a set of Koni coilover shocks from Factory Five.

The installation required some planning, and this is not a bolt-in by any means. The cradle is a close fit to the Comet's subframe, but not close enough, so each mount was fabricated under the car. We did not need a tubing bender; all of the mounts are made from thick-wall square tubing that we sourced at the local metal supply shop. As with any fabrication project, cut once, measure, cuss like a sailor, and then cut it again (or something like that—Ed.).

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There were a few unexpected issues we found during the install. One in particular is the upper-control-arm-to-subframe clearance; there isn't any. With the upper arm installed, it comes into contact with the subframe before ride height. To fix this, we simply notched the subframe using a piece of 4x5 square tubing. Another issue turned out to be the factory tolerances. The 1960s manufacturing processes were notorious for poor tolerances, and there were several areas that were not the same side to side on the wagon. In order to keep it tracking straight, we needed verifiable reference points on each side. In the end, we measured from several points on the car.

Depending on what vehicle the donor parts come from, you could end up with an open differential. We didn't know the specs of the 8.8 that we had, so we chose to rebuild it with parts from Randy's Ring and Pinion, adding a Yukon Sure-Grip LSD and a set of 3.73 gears. Those parts are not included in the project total, though, as they are not required for every swap.

Aside from the basic handtools, you will need a MIG welder, chop saw, reciprocating saw, and a plasma torch if you can get one. The plasma torch is really nice when working with some of the tight spots and thick metal, but you can do it without one. A lift is really helpful as well. We put the Comet on our Quality Lifts 4-post lift and the added headroom under the car made this project much easier than lying on the ground and dropping hot slag on your stomach.

One other note—with the cradle installed, fitting the center section is a tough task. There is a sweet spot on the passenger rear section of the cradle where the pumpkin can be rolled into the cradle. Keep that in mind for your swap. The 8.8 weighs about 80 pounds, so it takes four hands to move it around in this tight spot.

For the running gear, we picked up a set of Weld Racing S71 18s and 20s, wrapped in BF Goodrich G-Force Radial TA KDW rubber. The lightweight forged wheels look ridiculously good against the Comet body and the large open spokes provide ample cooling area for the brakes.

This swap will work on any early Mustang or Falcon platform, and the total price is less than any custom-fit IRS kit, ringing the register at $2,600 for the cradle and T-bird IRS components, we replaced the hub bearings and rotors for an additional $250, bringing the total to $2,850, under our $3,000 goal.

We spent about four days installing the cradle into the Comet, and the results could not be more impressive, on the ground, you can't tell the wagon has an IRS, and who would even think it would?